In transmission computed tomography, a patient or subject cross-section of interest is successively scanned from a number of directions by an x-radiation source to direct X-rays through a cross-section of interest. One or more detectors positioned on an opposite side of the patient obtain intensity readings of the x-radiation after it has passed through the patient. If enough intensity measurements from different directions are obtained, these intensity readings can be utilized to reconstruct a density mapping of the patient cross-section.
Computed tomography reconstruction techniques are derived from mathematical reconstruction algorithms utilizing the fact that each radiation intensity reading corresponds to a line integral of an attenuation function taken through the patient cross-section from the source to the position the intensity is sensed. These reconstruction algorithms allocate this attenuation along the path the radiation takes in traversing the patient in a process known as back projection.
Fourth generation computed tomography designs include an array of stationary detectors and a moving x-radiation source. The fourth generation array of detectors typically surrounds a patient aperture which defines a patient scanning plane. An X-ray source then radiates the plane from a number of different directions. This scanning is typically achieved by orbiting an X-ray tube about the patient and detecting X-ray intensities of radiation passing through the patient.
In early computed tomography scanner designs the detectors also moved as the patient was scanned. In orbital CT designs, for example, the X-ray source and an arc of detectors orbit in unison about the patient.
A common need in all commercial transmissive computed tomography scanners known to applicants is a motive force for moving at least the X-ray source and in some designs both the source and detectors. A motor for rotating the X-ray source must be capable of applying a large torque to a fairly large X-ray gantry to accelerate the gantry in a short time to a constant rate of rotation.
In currently available CT equipment such as a fourth generation CT scanner designated the Synerview 1200 which is commercially available from Picker International, Inc. of Cleveland, Ohio, the rotating CT apparatus defines an aperture of sufficient diameter to allow a patient torso to be inserted for scanning. Since the inertia of any rotational apparatus increases with distance from the axis of rotation, provision for full body scanning capability results in rotating apparatus (including an X-ray tube) having a large inertia.
When a Synerview 1200 or other scanner is used, a motor is energized, a scan taken and motion stopped. Typically, the direction of rotation is then reversed and a second scan is taken. The direction of scan rotation alternates back and forth so that only a finite angle of scanning motion is traversed each scan.
The scanning apparatus requires a braking force be applied after each CT scan and because of the high inertia, the required force is large. The Synerview 1200 scanner uses dynamic braking by reverse energizing its motor. Shock absorbing stops in the Synerview 1200 limit the travel of the CT rotating frame in the event the dynamic braking fails or is inadequate. Without the shock absorbing stops the cabling leading to the high voltage X-ray tube will be damaged in the event of a failure of dynamic braking to stop scanning motion.